1. Field of the Invention
The invention relates to organic semiconductor devices.
2. Discussion of the Related Art
Field-effect transistors (FETs) with organic semiconductors as active materials are the key switching components of contemplated organic control, memory, or logic circuits, also referred to as plastic-based circuits. An expected advantage of such plastic electronics is the ability to fabricate them more easily than traditional silicon-based devices. Plastic electronics would thus provide a cost advantage in cases where it is not necessary to attain the performance level and device density provided only by silicon-based devices. For example, organic semiconductors are expected to be much more readily printable than vapor-deposited inorganics, and are also expected to be less sensitive to air than recently-proposed solution-deposited inorganic semiconductor materials. For these reasons, there have been significant efforts expended in the area of organic semiconductor materials and devices.
Generally, in fabricating such FETs, the organic semiconductor is formed as a continuous film covering many devices simultaneously or even covering the entire circuit. (An organic semiconductor film is an assembly of organic molecules or polymerized organic monomers formed such that the film is capable of being active in a semiconductor device, e.g., by allowing current to flow between source and drain electrodes with use of a gate.) It is often desirable to have the semiconductor film present only at selected regions of a circuit, or selected regions of an individual device, to achieve, for example, lessened leakage current and crosstalk. Such selective formation also enables the use of different semiconductors in different regions of a device or in different devices of a circuit.
Various methods for patterning deposited films have been developed. For example, it is possible to use established photoresist technology to pattern a deposited organic film. Unfortunately, conventional steps such as reactive ion etching and solvent development tend to leave unwanted residue and also have the potential to degrade the semiconductor film. As an alternative, for some materials it is possible to selectively deposit, e.g., print, a solution of an organic semiconductor onto a substrate such that a semiconductor film is formed upon evaporation of the solvent. Unfortunately, such solutions are often not viscous enough to hold a desired shape upon printing, and may spread or contract depending on the surface forces. In addition, such solutions often are not concentrated enough to form an adequate film after a single printing step, and thus multiple deposition steps are required.
Improved techniques for providing patterned organic semiconductor films are therefore desired.
The invention provides an improved process for forming devices utilizing patterned organic semiconductor films. The process involves treating a surface to selectively provide regions of greater affinity and lesser affinity for an organic semiconductor or an organic semiconductor solution (i.e., an organic semiconductor material in a solvent). When the organic semiconductor, or solution comprising the semiconductor, is deposited on the treated surface, either the organic semiconductor or the organic semiconductor solution dewets from the lesser affinity regions or the resultant film adheres only weakly to the lesser affinity regions such that selective removal is readily performed. And even where such removal is not performed, the portions of the organic semiconductor film overlying the greater affinity regions exhibit higher mobility and better film continuity relative to the other portions of the film.
Regions of greater affinity are generally created by providing a surface with moieties that interact favorably with the organic semiconductor material to be deposited thereon (or its solvent in the case of a solution). Regions of lower affinity are generally created by providing a surface with moieties that are less compatible with moieties on the organic semiconductor material (or its solvent in the case of a solution). It is possible for the surface treatment to involve providing an affinity-increasing material in selected areas, such that the remaining surface constitutes the lower affinity regions. It is also possible for the treatment to involve providing the surface with an affinity-reducing material in selected areas, such that the remaining surface constitutes the greater affinity regions. Other combinations are also possible.
In one embodiment, the organic semiconductor film is formed over the entire treated surface. Portions of the film overlying regions of low affinity are then removed. The differences in affinity between the greater affinity regions and lower affinity regions allows such removal to be readily and selectively performed. In another embodiment, a semiconductor solution selectively and spontaneously wets primarily, or even exclusively, the regions of greater affinity prior to solvent evaporation. The resultant film is therefore present primarily, or exclusively, on the regions of greater affinity, with no need to remove undesired portions of the film.